Slide resistant platform anchor conductor silo

ABSTRACT

A platform anchor and conductor silo for a slide resistant offshore platform comprises at least two large diameter concrete cylinders joined end to end, said concrete being longitudinally prestressed by a plurality of high strength steel tendons, said platform anchor having removable end closures. The anchor is towed to location and upended with the large diameter end down. When the internal pressure is increased to an amount equal to the external hydrostatic pressure at the lower end, the lower end closure is removed. The anchor is then positioned over the desired location, the internal pressure is reduced, causing an increase in effective weight forcing the anchor into the ocean floor. When the upper closure is submerged and the internal pressure further reduced below hydrostatic, the resulting pressure imbalance provides additional force to drive the anchor into the ocean floor to a substantial depth.

[75] Inventor:

United States. Patent [191 Brasted SLIDE RESISTANT PLATFORM ANCHORCONDUCTOR SILO [52] U.S. Cl 6l/46.5, 6l/53.5, 61/82, 1 14/206 R [51]Int. Cl... E02b 17/00, E02d 23/02, B63b 21/26 [58,] Field of Search61/46, 46.5, 52, 50, 53.5, 61/53.56, 53.68, 53.64; 52/725, 229, 423; 114/206 R [56] References Cited v I UNlTED STATES PATENTS 3,643,4472/1972 Pogonowski.... 61/46 3,496,900 2/1970 Mott et a1 114/206 R3,620,026 11/1971 Mallard 6l/53.5 3,512,811 5/1970 Bardgette et a1.61/46 1,069,328 8/1913 Griffin 52/423 844,294 2/1907 Winslow 61/533,624,702 1l/1971 Meheen 61/46 3,613,381 10/1971 Cox.... 61/46.53,483,708 12/1969 Marshall 6l/53.5 3,380,256 4/1968 Rebikoff 6l/46.53,138,932 6/1964 Kofahl et a1. 6l/46.5 1,706,002 3/1929 Sipe 61/53.62

OTHER PUBLICATIONS Caisson Piers Designed For Offshore Oil Drill Plat-5] Apr. 23, 1974 Concrete Pile Supports A reprint of an article fromOffshore Magazine Furnished By Raymond Inc. 140

Cedar St., N.Y., NY. 10006 Primary Examiner-W.C. Reynolds AssistantExaminer-Alexander Grosz [5 7] ABSTRACT A platform anchor and conductorsilo for a slide resistant offshore platform comprises at least twolarge diameter concrete cylinders joined end to end, said concrete beinglongitudinally. prestressed by a plurality of high strength steeltendons, said platform anchor having removable end closures. The anchoris towed to location and upended with the large diameter end down. Whenthe internal pressure is increased to an amount equal to the externalhydrostatic pressure at the lower end, the lower end closure is removed.The anchor is then positioned overthe desired location, the internalpressure is reduced, causing an increase in effective weight forcing theanchor into the ocean floor. When the upper closure is submerged and theinternal pressure further reduced below hydrostatic, the resultingpressure imbalance provides additional force to drive the anchor intothe'ocean floor to a substantial depth.

6 Claims, 9 Drawing Figures pmmmmza 1914 3805584 sum 1 or 2 FIG]BACKGROUND OF THE INVENTION The invention relates to a method andapparatus for anchoring an offshore drilling or production platform indeep water where soil slides frequently occur making producingoperations extremely hazardous due to the shearing action of the soilacting on the platform support legs and well conductors.

DESCRIPTION OF PRIOR ART The oil industry in its efforts to locate andproduce hydrocarbons at offshore locations, particularly inthe Gulf ofMexico, is continually confronted with conducting drilling operations ingreater water depths as well as in areas where underwater mud slidesoccur causing extensive damage to offshore structures anchored in thefloor of the body of water.

The prior art suggests several methods of sinking caissons in the floorof the ocean. US. Pat. jNo. 2,475,888 illustrates a pier capable ofbeing embedded in the ocean floor by means of rotation, the pier pullingitself down into the ocean floor through the use of enormous screwthreads disposed on the outer diameter of the pier. A'method such' asthis would be unwieldy and unworkable at water depths of 300 feet.Another method used in installing a caisson in the ocean floor isillustrated by US. Pat. No. 3,380,256 which shows a large diametercaisson used in the ocean floor having minimal penetration into theocean floor and extending above the ocean floor a substantial distance,i.e., at least the height of a drilling rig.

It will be noted that both of these patents illustrate caissons thateither extend to the surface of the body of water or at least extend asubstantial distance above the ocean floor.

It-will be further noted that US. Pat. No. 3,380,256 also disclosessinking of the caisson by having the upper end closed and partiallyevacuating the inside volume of the caisson thus utilizing thehydrostatic head of the water to aid both in sinking the caisson andthereafter to aid in driving it into the ocean floor a minimal depthwhereupon drilling operations are carried out on the ocean floor withinthe caisson.

SUMMARY-OF THE INVENTION without having to assemble component partsafter reaching the offshore location.

DESCRIPTION OF THE DRAWING The nature of the present invention will morefully be appreciated with reference to the drawing in which:

FIG. 1 illustrates the slide resistant anchor and conductors as usedwith an offshore platform;

FIG. 2 shows an overall view of the combined platform anchor andconductor silo; FIG. 3 is a crosssectional view taken along line 33 ofFIG. 2;

2 FIG. 4 is a cross-sectional view taken along line 4-4 of FIG. 2; and

FIGS. 5-9 illustrate a preferred method of installing the anchor andconductor silo of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, andmore particularly to FIGS. 24, one embodiment of the combination anchorand conductor silo 10 is shown. The anchor 10 is fabricated inindividual cylindrical sections of approximately 10 to 30 feet in lengthand then aligned end to end for assembly. The small diameter or upperend section 2 is provided with a plurality of conduits 41 about itscircumference through which high strength steel tendons 3 are installedand suitably anchored at one end (FIG. 4) to a connector 1 for changingdiameters. A tensioning force is then applied to the opposite end tomaintain the tendons 3 and the members 2 at the desired levels of stressbefore securing the tendons 3 to the uppermost concrete member 2. Thelarger diameter or lower cylindrical members 5 are similarly providedwith a plurality of conduits 42 through which high strength steeltendons 6 are installed and tensioned as described for members 2. Thediameters of the members 2 and 5 are large compared to the wallthickness of the cylindrical concrete members 2 and 5. For example, theinternal diameter of cylindrical member 2 may be from 19 to 20 feetwhile having a wall thickness of approximately 2 to 3 feet and thecylindrical member 5 may have an internal diameter of 30 to 40 feet anda wall thickness of roughly 3 to 5 feet.

Prior to being launched the removable end caps 7 and 8 are installedclosing the ends of anchor 10 enabling it to float in water 15 as shownin FIG. 5. Removable straps 9 are installed so that buoyant floats 11may be suitably attached to provide stability to the anchor 10 while itis towed to location by a conventional means 12. The floats 11 andstraps 9 are removed from the anchor 10 upon arrival at location.

The anchor 10 is then upended using the derrick barge 12 (FIG. 6) suchthat the large diameter end of anchor 10 is nearest the ocean floor20.The cable means 16 connected to end closure 8 and barge 12 is used tosuspend the anchor 10 relative to the floor 20 as shown in FIG. 6. Apump means 13 on barge 12 is in fluid communication with the inner boreof the anchor 10 through a flexible hose means 14 and is used topressurize the inside of the anchor 10, thereby forcing removable endclosure 7 from the lower end (FIG. 7). After removal of the end closure7 the anchor 10 is positioned over the final location and lowered intocontact with floor 20. The hose means 14 is then utilized to bleedpressure from the inside of the anchor 10 to increase its effectiveweight aiding the anchor 10 in penetrating the floor 20. Furtherreduction of pressure inside the anchor 10 below the hydrostatic headabove the anchor 10 provides a penetrating force which in combinationwith the dead weight of the anchor 10 drives the anchor 10 to thedesired depth D (FIG. 9), which may be on the order of 200 to 350 feetbelow the sea floor 20. Penetration of the anchor 10 into the sea floor20 is preferably to a depth corresponding to approximately the fulllength of the anchor 10. The total length of the anchor 10 can bereadily adjusted as con- 4 ditions of soil instability dictate.Preferably the method taught by the present invention is used in waterdepths equal to or exceeding 250 feet. Penetrating to a depth D as shownin FIG. 9 greatly improves the ability of an offshore platform (FIG. 1)to withstand the forces developed in areas where soil slides are apotential hazard. In areas such as this, soil instability may beprevalent to depths of 1'00 feet or more below sea floor 20 making itessential that any anchoring means extend to a substantially greaterdepth.

After the anchor 10 has been installed to the desired depth D, the endclosure 8 is removed through the use of cable 16 and barge 12. Aprotective steel cap sleeve 21 is then installed at the upper end ofanchor 10 using cable 16 and barge 12. The protective steel sleeve 21 issubsequently grouted to the anchor 10. Additional anchors 10, ifrequired, are installed using the previously described procedure priorto installing a platform structure 30, such as that shown in FIG. 1.Such a platform structure 30 is well known in the art and has aplurality of legs 25 extending substantially vertically upwardly fromthe ocean floor 20 with accompanying structure. Suitably attached to theprotective sleeve 21 are horizontal support members 26 that extendoutwardly and are connected to legs 25 and other cross members 27. Afterthe platform assembly 30 has been completely installed and connected tothe anchors 10, the inside of anchors 10 then act as conduits forsubterranean wells drilled into the ocean floor 20. Thus, the anchor 10also serves to protect the conductor pipes 31 from being subjected toshear forces incurred during soil slides.

in one embodiment of the invention the anchor 10 would have thefollowing nominal properties:

Outside Diameter Inside Diameter Length Upper End 25 feet 19.5 feet 105feet Lower End 35 feet 27.0 feet 270 feet An anchor 10 having thesedimensions would have a total weight of approximately 10,035 tons and atotal displacement of 10,082 tons. By evacuating the inside of theanchor a force of 5,700 tons due to its own weight plus the hydrostatichead acting upon the cross sectional area of the anchor 10 provides thedriving force required to penetrate the sea floor 20 to a substantialdepth, D.

If more force is required to drive the anchor into the sea floor thanthe combined anchor weight and hydrostatic head, additional temporarylength of anchor may be attached to the primary anchor. These temporarylengths may extend to the surface of the water or to a substantialdistance above it. The temporary anchor extension could be bridged-offat the bottom and filled with a weighting material such as drilling mud.Depending on the volume of the extension and the specific gravity of theweighting material, the driving force can be controlled and varied overa wide range.

In a second embodiment of the present invention, steel cylinders aresubstituted for the concrete members 3 and 5. An anchor using steelcylinders would be of a constant internal diameter and would have a wallthickness of approximately 2-5 inches. The same method would be utilizedto install an anchor made from steel cylinders as is used for an anchormade from concrete members.

While only one embodiment of the invention has been illustated it is notlimited to only one but is intended to disclose both a method andapparatus for anchoring an offshore platform in areas where soilinstability is prevalent.

I claim as my invention:

1. A method of installing an anchor having removable end closures in thefloor of a body of water to anchor an offshore platform, said methodcomprising the steps of:

upending said anchor such that said anchor is in a substantiallyvertical position;

removing the end closure adjacent the floor of said body of water;without removing the other end closure;

positioning'said anchor so that the bottom of said anchor is in contactwith the floor of said body of water;

evacuating the interior portion of said anchor creating a pressuredifferential between said interior portion and the exterior of saidanchor; forcing the anchor into the floor to a depth approximately equalto the full length of the anchor;

removing the top end closure from said anchor; and installing protectivesleeve means at the upper end of said anchor.

2. The method of claim 1 wherein the end closure adjacent the floor ofsaid body of water is removed by suitably attaching said platform tosaid anchors.

6. The method of claim 5 wherein said anchors act also as conductorsilos for wells drilled into the floor of said body of water.

1. A method of installing an anchor having removable end closures in thefloor of a body of water to anchor an offshore platform, said methodcomprising the steps of: upending said anchor such that said anchor isin a substantially vertical position; removing the end closure adjacentthe floor of said body of water; without removing the other end closure;positioning said anchor so that the bottom of said anchor is in contactwith the floor of said body of water; evacuating the interior portion ofsaid anchor creating a pressure differential between said interiorportion and the exterior of said anchor; forcing the anchor into thefloor to a depth approximately equal to the full length of the anchor;removing the top end closure from said anchor; and installing protectivesleeve means at the upper end of said anchor.
 2. The method of claim 1wherein the end closure adjacent the floor of said body of water isremoved by pressurizing the interior of said anchor.
 3. The method ofclaim 1 wherein the anchor is forced into the floor of said body ofwater to a point below the region of said instability.
 4. The method ofclaim 1 wherein a second anchor is installed a fixed distance from theanchor installed by the method of claim
 5. The method of claim 4 furtherincluding the steps of: installing an offshore platform over saidanchors; and suitably attaching said platform to said anchors.
 6. Themethod of claim 5 wherein said anchors act also as conductor silos forwells drilled into the floor of said body of water.